1. Field of the Invention
This invention relates to a means which, when a plug mates with a receptacle, prevents outside water from entering into the coupling between the plug and the receptacle.
2. Description of the Prior Art
In a typical structure of currently used watertight plugs and receptacles for an electric connection, a threaded lock portion is formed along the outer periphery of the plug-receiving portion of the receptacle, and a threaded metallic lock member is mounted on the body portion of the plug so as to be turned about the axis of the body portion, and a packing is mounted within the plug so as to surround plug-blades and to extend in the direction of said axis at right angles to the plug-blades. After the plug-blades are inserted in the receptacle, the metallic lock member is turned for making the plug-receptacle coupling watertight. This conventional structure has a shortcoming in that, if the metallic lock member is not fully tightened, reliable watertightness of the plug-receptacle coupling cannot be ensured, and that, if a blind cover is to be mounted when the plug-receptacle coupling is not used, the mounting of such blind cover becomes complicated.
In another structure of conventional watertight plugs and receptacles, a packing acting as a watertight belt is disposed between the outer periphery of the support portion of a plug and the inner periphery of the plug-receiving support portion of a receptacle, so that when the plug mates with the receptacle, the packing tightly engages both of the two support portions for providing watertightness. This structure provides comparatively good watertightness, but the two support portions are difficult to couple and separate, and if the coupling is loose, the capillary phenomenon takes place between the packing and the contact surface of the support portions, so that the degree of watertightness is reduced. On the other hand, to achieve high watertightness, the two support portions must be tightly coupled, which tight coupling is difficult to make, and the two support portions coupled at a high watertightness are susceptible to accidental separation by the pressure of air trapped therebetween when the plug-receptacle coupling is vibrated. Thus, there is a shortcoming in that the separability of the two support portions depends on the degree of watertightness.